Copper-containing metallic effect pigments, comprising copper pigments or brass pigments produced from a copper-zinc alloy, also referred to as gold bronze pigments, are used in industries including the graphics industry, as for example in printing inks.
The metallic effect pigments used to date for pigmenting flexographic and gravure inks and produced by milling from copper or brass powder, such as “Rotovario” gold bronze pigment dispersions or “Rotoflex” stabilized leafing gold bronze pigment powder or “Rotosafe” stabilized leafing gold bronze pigment pellets from Eckart GmbH, D-90763 Fürth, Germany, for example, are of only limited suitability for the pigmentation of film reverse applications with mirrorlike effect, on account of their leafing properties. The aforementioned copper-containing metallic effect pigments are obtained by dry milling of copper or brass powder.
The use in printing inks of brass pigments produced by PVD methods is also problematic in as much as homogeneous metallization of the two metals (copper and zinc) with very different vaporization temperatures in a high vacuum is extremely difficult from a technical standpoint, with the aim of achieving a uniform hue. Moreover, unlike the brass pigments produced by conventional milling, the PVD brass pigments, which are relatively expensive to produce, have layers which are not very compact, with densities below the densities of the respective materials, and the desired gold hues (especially rich gold) cannot be realized at the low layer thicknesses desired.
EP 1 529 084 B1 describes gold bronze pigments which can be produced by PVD methods. On account of the complex method, these pigments are very expensive. Moreover, these pigments tend toward partial phase separation of the alloying constituents, which is likewise accompanied by unwanted shifts in hue and by inadequate stabilities of hue.
GB 994,409 discloses a varnish, ink and paint composition which comprises copper-containing pigments and benzotriazole as corrosion inhibitor. The benzotriazole prevents discoloration of the varnish, paint or ink composition caused by oxidation of copper.
According to the teaching of EP 08009699.3, unpublished at the priority date of the present specification, copper-containing, platelet-shaped metallic effect pigments having improved optical properties are obtained when these pigments have a thickness distribution, determined by scanning electron microscopy (SEM) and represented as cumulative undersize distribution, with an h50 of 10 to 50 nm and an h90 of 20 to 70 nm.
It has now emerged that the copper-containing metallic effect pigments produced in accordance with the teaching of EP 08009699.3, on contact with binders of a coating composition, such as varnish, paint, printing ink, etc., may result in an unwanted increase in the viscosity of the varnish, paint or printing ink. This increase in viscosity of a coating composition, as of a printing ink, for example, which may also be referred to as gelling of the coating composition, as of the printing ink, for example, may take place within a few hours or days. A coating composition, as for example printing ink, which has undergone such gelling or is of high viscosity can no longer be applied—printed, for example—at all, or only with loss of the high-grade optical properties. If its viscosity is very high, the coating composition, as for example printing ink, can no longer be dispersed with addition of solvent, or else a very large quantity of solvent must be added, resulting in an impairment in quality in relation to the optical properties of the coating composition. In the case of a printing ink, for example, an increased quantity of solvent may lead to a significant detraction in the context of ink transfer during printing.
There is therefore a need for platelet-shaped, copper-containing metallic effect pigments which have outstanding optical properties, especially in the context of film reverse applications, and which do not give rise to an unwanted increase in viscosity following introduction into a coating composition.